AC of -15 or greater and fire resistance is recommended when facing Ixoth. He gets many powerful attacks, but will miss often if your AC is around -15. He is difficult to wake up because of his dark cave, and will usually teleport next to you to start fighting when you're walking around the bend on the bottom part of his cave. He respects Elbereth, and will recover his HP on the upstair if you give him a chance. If your lance is skilled or expert and you still have a mount, you may be able to joust him to death. If not, you'll probably have to engage him, return to the upstair, lure him off, get on it yourself, and beat him to death.

In the West the dragon was the natural enemy of man. Although
preferring to live in bleak and desolate regions, whenever it
was seen among men it left in its wake a trail of destruction
and disease. Yet any attempt to slay this beast was a perilous
undertaking. For the dragon's assailant had to contend
not only with clouds of sulphurous fumes pouring from its fire
breathing nostrils, but also with the thrashings of its tail,
the most deadly part of its serpent-like body.

[ Mythical Beasts by Deirdre Headon (The Leprechaun Library) ]

"One whom the dragons will speak with," he said, "that is a
dragonlord, or at least that is the center of the matter. It's
not a trick of mastering the dragons, as most people think.
Dragons have no masters. The question is always the same, with
a dragon: will he talk to you or will he eat you? If you can
count upon his doing the former, and not doing the latter, why
then you're a dragonlord."